It is known that aldehydes and alcohols can be prepared by reaction of olefins with carbon monoxide and water. This reaction, which is called hydroformylation (oxo synthesis), is generally catalyzed by hydridometalcarbonyls, preferably those which are derived from metals of group VIIIA of the Periodic Table (IUPAC Version).
In addition to cobalt, which is widely used industrially as the catalyst metal, rhodium has recently been acquiring increasing importance. In contrast to cobalt, rhodium enables the reaction to be carried out under a low pressure; moreover, straight-chain aldehydes are preferentially formed, and branched aldehydes are formed to only a minor degree. Finally, hydrogenation of the olefins to give saturated hydrocarbons is significantly lower when rhodium catalysts are used than when cobalt catalysts are used. In the processes introduced in industry, the rhodium catalyst is employed in the form of modified hydridorhodiumcarbonyls, which additionally contain, in excess if appropriate, ligands. Tertiary organic phosphines and phosphites have proven to be particularly suitable ligands.
The catalyst system consisting of HRh(CO) (PPh.sub.3).sub.3 and excess PPh.sub.3 (Ph.dbd.C.sub.6 H.sub.5) is thus employed, for example, for hydroformylation of alpha-olefins, such as ethylene, propylene and butene-1 at temperatures between 90.degree. and 120.degree. C. This system and reaction are disclosed in U.S. Pat. No. 3,527,809.
The outstanding suitability of the Rh/triphenylphosphine catalyst for the hydroformylation of alphaolefins is confirmed in European Patent 149,894 B1. At the same time, however, it is pointed out that the reaction of internal olefins such as butene-2 presents problems, because it leads to significant conversions only if higher temperatures are used. Under these conditions, however, isomerization of the olefin increases greatly. Some of the butene-2 is thus converted into butene-1, with the result that, in addition to the desired 2-methylbutyraldehyde, n-valeraldehyde is also formed to a considerable extent in the hydroformylation. At the same time, the activity and stability of the catalyst system also decrease, as is taught by European Patent 96, 987 B1 by the hydroformylation of butene-2. The rhodium/triphenylphosphine catalyst system is therefore unsuitable for the hydroformylation of internal olefins on an industrial scale.
The difficulties described above can be avoided if trialkyl or triaryl phosphites are employed as the catalyst component instead of trialkyl- or triarylphosphines. Organic phosphites in fact have the advantage that the hydroformylation of olefins proceeds at lower temperatures in their presence than if organic phosphines are used. Thus, according to Example 1 of European Patent 96,988 B1, butene-2 reacts with carbon monoxide and hydrogen under 2.86 MPa at 98.5.degree. C. to give 2-methylbutyraldehyde using rhodium and cyclic phosphites as the catalyst.
German Specification 1,793,069 B2 describes the preparation of aldehydes by hydroformylation of olefins in the presence of rhodium compounds (which contain carbon monoxide bonded as a complex) and a triaryl, trialkyl or tricycloalkyl phosphite as the ligand, there being at least 2 mol of free ligand per g atom of rhodium present in the reaction medium. Examples therein relate both to the reaction of olefins and olefinically unsaturated compounds. Alkyl and aryl compounds are used as the phosphites.
The hydroformylation of 3,3-dialkoxy-1-propenes is the subject matter of German Specification 34 03 427 A1.
According to the procedure claimed, 3,3-diethoxybutanal and 2-methyl-3,3-diethoxypropanal are obtained in a molar ratio of 8.5 to 1 from for example 3,3-diethoxy-1-propene in the presence of Rh/triphenyl phosphite at 110.degree. C. under a pressure of 0.3 MPa; the conversion is 99.5%.
According to European Patent 3,753 A1, rhodium, together with triphenyl phosphite, is likewise used as the catalyst in the reaction of cyclic acrolein acetals with carbon monoxide and hydrogen to give the corresponding aldehydes.
The hydroformylation of alpha, beta-unsaturated nitriles is described in U.S. Pat. No. 4,344,896. The reaction is carried out in the presence of rhodium which contains carbon monoxide, bonded as a complex, and, inter alia, an organic phosphorus compound. The phosphorus compound can be a phosphite, such as triphenyl phosphite, tri-4-tolylphosphite, tri-4-chlorophenyl phosphite, triethyl phosphite or tributyl phosphite.
Numerous publications relate to the use of specific phosphites as a constituent of hydroformylation catalysts.
The content of linear aldehydes as products of the hydroformylation of alpha- or beta-olefins is particularly high if rhodium complex compounds which contain fluorinated organic phosphites as ligands are employed as catalysts. This is taught in U.S. Pat. No. 4,330,678.
According to U.S. Pat. No. 4,467,116, less reactive olefins are hydroformylated in the presence of a catalyst which is a metal of group VIII A, modified, inter alia, by a triaryl phosphite as the ligand. At least one aryl radical of the phosphite is substituted by an optionally fluorinated alkyl group or by an aryl group.
Van Leeuwen and Roobeek in J. Organomet. Chem. 258 (1983), 343 et seq. also deal with the hydroformylation of non-reactive olefins, such as 2-methyl-1-hexene, limonene, cyclohexene and methylenecyclohexane. The reaction of such olefins under mild conditions (90.degree. C. and 1 MPa) proceeds in the presence of rhodium catalysts modified by phosphite. Examples of the phosphite ligands employed are tris(o-t-butylphenyl) phosphite and tris-(hexafluoroisopropyl) phosphite; they are distinguished by specific steric and electronic properties.
Although the organic phosphites have a number of advantages over the organic phosphines as constituents of catalysts for the oxo synthesis, they are used to only a limited extent industrially. Their limited use is to be attributed to the fact that the activity of rhodium/phosphite catalysts decreases in the course of time, especially if they are used in the upper region of the particular temperature range recommended. At the same time, higher-boiling compounds are formed to an increased extent. Side reactions which convert the organic phosphite into inactive secondary products are at least partly the cause of both phenomena.
In this connection, it should be remembered that phosphorous acid triesters are very sensitive to hydrolysis. The traces of water formed by reduction processes during hydroformylation are sufficient to hydrolyze the triesters to di- and monoesters and to free phosphorous acid. Moreover, the acid mono- and diphosphites catalyze the hydrolysis of the triester, so that the reaction proceeds autocatalytically.
The rate of hydrolysis depends greatly on the nature of the ester radicals. Trimethyl phosphite is the most unstable and, as the length of the alkyl radicals increases, the phosphites become more resistant to hydrolysis. The activity of the catalyst system is furthermore impaired by the fact that the acid monoester is capable of protonating the rhodium/phosphite complex, i.e. converting it into a form which is likewise catalytically inactive.
The hydrolysis of the phosphorous acid ester can be prevented or at least certainly delayed by addition of organic or inorganic bases to the reaction mixture. According to the process of European Patent 285,136 A1, for example, secondary organic phosphites are removed selectively from solutions containing them together with tertiary organic phosphites by addition of an amine and removal of the ammonium phosphite formed. This procedure cannot be used unreservedly in the case of hydroformylation of olefinic compounds in the presence of rhodium/phosphite catalysts. It requires the addition of a substance foreign to the reaction to the reaction mixture, which can give rise to undesirable secondary reactions.
Further losses in phosphite may occur by reaction of the phosphorus compounds with aldehydes. As F. Ramirez demonstrated (Synthesis, 1974, 90 et seq.), phosphorous acid triesters form 4,4,4-trialkoxy-1,3,4-dioxaphospholanes with aldehydes at low temperature, and predominantly 2,2,2-trialkoxy-1,3,2-dioxaphospholanes at elevated temperature. Both classes of compounds are catalytically inactive; their formation influences the ratio of rhodium to phosphite in the catalyst system and, in this way, leads to a reduction in the activity of the hydroformylation catalyst.